Diesel engine combustion exhaust includes various emissions, such as carbon dioxide, carbon monoxide, unburned hydrocarbons, NOx, and particulate matter (PM). Increasingly, environmental regulations call for emissions controls to aggressively lower diesel exhaust emission levels for NOx and PM. These standards include, for example, EURO 4 (2005) and EURO 5 (2008) and U.S. Year 2004 and U.S. Phased 2007-2010 Emissions Limit Standards. Regulations are increasingly limiting the amount of NOx that can be emitted during a specific drive cycle, such as the FTP (Federal Test Procedure) in the United States or the MVEG (Mobile Vehicle Emission Group) in Europe.
One of the ways known in the art to remove NOx from diesel engine exhaust gas is by catalyst reduction. The catalyst reduction method essentially includes passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. For example, known emission reduction systems include systems for supplying fuel oil as hydrocarbon (HC) reductant or ammonia provided in the form of urea, either of which are injected into the exhaust gas upstream of the NOx catalyst.
Another way to remove NOx from diesel exhaust gas is by use of a NOx absorber catalyst. In this case, NOx is trapped in the absorber catalyst as the exhaust gas stream passes therethrough. Over time, the absorber catalyst may become saturated with NOx. To alleviate such a condition, the absorber catalyst is periodically regenerated to remove the NOx from the absorber catalyst by converting the trapped to NOx to nitrogen.
Diesel particulate filters (DPF) for the removal of PM from a diesel engine exhaust stream have been proven effective to remove carbon soot. A widely used DPF is the wall flow filter which filters the diesel exhaust by capturing the particulate material on the porous walls of the filter body.